Corn grower’s guide for evaluating crop damage and replant options

What you need to know

The seven factors for evaluating whether to replant:

  1. The existing plant stand.

  2. Distribution of the plant stand.

  3. Calendar date.

  4. Weed situation.

  5. Seed availability of earlier maturing hybrids.

  6. Cost to replant.

  7. Yield potential of the existing crop.

This guide helps Minnesota corn growers evaluate hail damage and other crop injuries, determine regrowth potential, and make informed decisions about whether to replant.

We use hail injury as the model, but the principles of evaluating crop damage and regrowth potential are similar to other crop injury situations.

Causes of crop injury

Corn crop injury occurs somewhere in the state every year. The major cause of crop injury is hail, which causes many millions of corn crop losses.

Insect feeding, flooding, low air temperature, nutrient deficiencies, crusted soils, soil borne fungi and bacteria, cold seedbeds or chemical injury (fertilizers, insecticides or herbicides) may also cause crop injury and reduced stands.

Replant factors to evaluate

Deciding whether to replant after crop injury is one of the most stressful and important decisions a farmer has to make.

Extreme injury to agricultural crops is far too common and can be an emotional event. But growers should make replant decisions based on facts from research rather than their “gut feelings” of anger and despair at the loss.

Here are the seven factors for evaluating whether to replant:

  1. The existing plant stand.

  2. Distribution of the plant stand.

  3. Calendar date.

  4. Weed situation.

  5. Seed availability of earlier maturing hybrids.

  6. Cost to replant.

  7. Yield potential of the existing crop.

When considering whether to replant the crop, the first two factors to evaluate are the plant stand and the distribution of the remaining plants.

Evaluating the plant stand

Establishing and maintaining an optimum plant stand is important for profitable crop yields.

Poor stands may occur for several reasons, such as poor germination, crusted surface (or other poor conditions for emergence), a cold seedbed, excess moisture, insects, chemical injury (fertilizers, insecticides or herbicides) or hail.

If hail caused the stand reduction, wait until three to five days after the storm so plants can begin to regrow. This gives time for some regrowth and, ultimately, a better evaluation of whether the plants will survive.

During this waiting time, growers can make all the necessary plans for replanting such as financial considerations; preparing the equipment; determining the availability of early-maturing, good-yielding hybrids and others.

If insects caused the stand reduction, growers can make the replant decision as soon as you find and evaluate the damage.

Growing point

When hail damages young corn plants, they usually regrow if the growing point remains healthy.

In corn, the growing point remains protected below the soil surface until the V5 stage (five collared leaves). Locate the growing point by splitting a stalk down the center.

If the growing point has been damaged, bacteria will often invade the plant and the growing point becomes brown and soft. These plants will not recover, so count them dead.

Leaves

Some plants severely damaged by hail may have difficulty regrowing.

Plants with leaves loosely bound in the whorl usually grow or blow out, and continue with normal development.

But plants with very tightly bound leaves in the whorl usually don’t. These plants are often referred to as buggy whips or ties (Figure 1). The leaves remain so tightly wrapped that some of the uppermost leaves and the tassel cannot emerge from the whorl.

It’s impossible to determine if these plants will recover, or the degree to which they will recover. Some of these tied plants might shoot an ear and produce some grain (tassel emergence is not necessary on each plant to allow pollination).

But do not count them as living plants when making the population count, as they are unlikely to significantly contribute to grain yield.

A healthy growing point will be white to light green in color, and firm in texture.

Young corn plant, wilted and brown lower leaves
Figure 1: Frost damage caused buggy-whipping in this Renville County corn plant.

Calculating the plant population

Measure the distance for one-thousandth of an acre for your row spacing, and count the number of live plants in that row section. Table 1 gives the length of row equivalent to one-thousandth of an acre for various row spacings. Then multiply by 1000 to determine the number of healthy plants per acre.

Make several checks throughout the field, because you may identify areas that do not need replanting.

Table 1: Length of row to equal one-thousandth of an acre for various row widths

Row spacing Row length
30 in. 17 ft., 5 in.
22 in. 23 ft., 9 in.
15 in. 34 ft., 10 in.

Optimum population

The optimum harvest population for achieving maximum grain yield is around 33,000 plants per acre for most corn hybrids and locations in the northern Corn Belt.

When the plant population drops below the optimum, most hybrids will increase in ear size (both kernel number and kernel size) and, sometimes, the number of ears per plant.

Small changes in plant population below the optimum result in larger kernels, and more kernels on the top ear. Large reductions in plant population result in more and larger kernels on the top ear, and significant grain production on the second ear.

So the reduction in grain yield is not directly proportionate to the reduction in stand (Table 2).

Uniformity

The uniformity of the remaining stand is also important. Uniformly spaced plants produce more per plant and more per acre, than unevenly spaced plants.

Clumped stands can vary in many ways that lead to non-uniformly spaced plants. Some parts of the field may have fairly uniformly spaced plants and near-optimum yields, while other areas have large gaps between plants within the row.

Large gaps reduce yield more than small gaps. Large gaps in the stand can lower grain yields by about 5 percent at plant populations between 14,000 and 28,000 plants per acre.

Add 5 percent to the yield reduction for lower-than-optimum populations with large gaps (2 feet or more) in the stand.

Table 2: Corn response to plant population

Final stand (plants/acre) Expected yield
44,000 100%
41,000 100%
38,000 100%
35,000 100%
32,000 100%
29,000 99%
26,000 96%
23,000 92%
20,000 87%
17,000 81%

When plant populations are lower than optimum and will no longer produce a maximum yield, compare the lower yield of late-planting a short-season hybrid with the yield potential of the reduced stand.

Also consider replanting costs, uncertainty of obtaining a good stand with a late planting and the possibility of yield reductions due to moisture stress at silking time.

It’s also important to remember that late-planted corn will have higher kernel moisture content at harvest, which will increases your cost to dry the grain to a storable moisture level.

Leaf loss and grain yield

Once you’ve decided you still have a sufficient number of live plants, determine the amount of leaf loss.

The defoliation amount and development stage at the time of the hailstorm will determine the effect on grain yield.

Completely defoliated young corn plants (up to the seven-leaf stage) usually results in little or no yield reduction. The older the plant, the more leaf loss will affect yield. This
has been determined by extensive research on a number of hybrids and locations in the
U.S. The results were used to establish the loss from leaf removal for hail insurance
adjustment (Table 3.).

Sometimes high-velocity winds or hail tears or shreds leaves. Leaf tissue remaining on the plant, and green in color, can continue to function and contribute to grain filling.

When determining the percentage of destroyed leaf area, only consider leaf tissue that’s completely removed or brown in color.

Table 3: How destroyed leaf area affects corn grain yield

*Leaf stage corresponds to number of leaves that are arched over and pointing downward.
Leaf stage* 20% of leaf area destroyed 40% of leaf area destroyed 60% of leaf area destroyed 80% of leaf area destroyed 100% of leaf area destroyed
7 0% yield loss 1% yield loss 4% yield loss 6% yield loss 9% yield loss
8 0% yield loss 1% yield loss 5% yield loss 7% yield loss 11% yield loss
9 0% yield loss 2% yield loss 6% yield loss 9% yield loss 13% yield loss
10 0% yield loss 4% yield loss 8% yield loss 11% yield loss 16% yield loss

Leaf loss and corn maturity

Leaf loss early in the growing season, particularly major amounts of leaf loss, is thought to set back the corn plant or delay maturity. But extensive research shows no appreciable delay in tassel emergence, silking date or kernel moisture content at harvest from partially or completely lost leaves between leaf stages 5 and 13.

Complete defoliation at these growth stages when the stalk is elongating causes significantly shorter plants. Plants can be as much as 8-10 inches shorter when completely defoliated at this time.

Corn will grow more slowly following leaf removal, depending on the amount of leaf area lost and the weather that follows. But the shorter plants that grow after defoliation are not “set back” in maturity.

Grain yield, of course, is reduced according to the amount of leaf area destroyed and the growth stage when the damage occurs.

Yield potential of late-planted corn

When growers plant corn in Late April or early May, yields are the highest and produce the most profit per acre.

Typically, corn yield rapidly declines when planting is delayed beyond mid-May. Table 4 gives the yield reduction associated with planting in late May and early June.

Table 4: How planting date affects corn grain yield in southern and central Minnesota

*Bruce Potter and Steve Quiring, University of Minnesota, personal communication.
Planting date Percent of maximum grain yield
Study 1: Lamberton (1988-2003)*
Percent of maximum grain yield
Study 2: Lamberton, Morris and Waseca (2009-2011)
20-Apr 99% 98%
25-Apr 100% 99%
30-Apr 100% 100%
5-May 99% 100%
10-May 98% 99%
15-May 95% 98%
20-May 92% 95%
25-May 87% 92%
30-May 82% 89%
4-Jun 76% 84%
9-Jun 69% 79%

For grain production, do not plant corn after June 15 in southern Minnesota, or after June 5 for central to northern Minnesota. Growers could plant corn for silage as late as June 25 in southern Minnesota. See recommended corn hybrid maturities for various planting dates and growing zones in Table 5.

Table 5: Recommended corn hybrid maturities for late planting

*For areas where 105 RM hybrids are full-season, plant hybrids with maturity ratings of 100 or less for grain production.
Planting date Relative maturity (RM) units earlier than full season*
May 25-31 5-7 relative maturity units
June 1-10 8-15 relative maturity units
June 11-15 15 or more relative maturity units

Making the replant decision

Using the information on plant population, leaf loss and late planting, you can compare the estimated losses from the hailstorm with the yield potential and costs associated with replanting the crop.

For the existing crop, consider the weed situation.

  • What is the population of weeds? The species?

  • Can they be controlled if the crop is left?

  • If not, what effect will they have on regrowth and yield potential of the existing crop?

  • And at what extra cost?

Replant costs

Replant costs including seed, labor and fuel, currently represent approximately 20 percent of the original crop potential. These costs are extra, so reduce the yield potential by 20 percent to pay the replant costs.

In addition to replant costs – which may vary greatly from farm to farm and year to year – be sure to include other real costs in the costs of replanting.

These include interest on loans taken to replant, and opportunity costs due to time spent replanting that could have spent on other profitable (or profit-saving) activities.

Replant costs may be partially or completely compensated if you have crop hail insurance that carries a replant clause. If you have insurance, notify your agent of your loss and ask about replant cost-sharing.

How to decide if replanting will pay

The following worksheet will help you decide whether it will pay to replant.

The option with the higher yield potential should be the more profitable approach. You will also need to consider the availability of seed, and replant costs for seed, labor and fuel.

Fill in the following worksheet and compare the yield potential of the existing crop with the yield potential of a replanted crop.

If you decide to replant, use Table 5 to choose seed with a relative maturity for your location and planting date.

Corn comparison worksheet

Field not replanted:

Estimated loss due to:

Reduced stand: ____ percent

Leaves removed: ____ percent

Weed Condition: Good, Fair or Poor

Sum of losses: ____ percent

Remaining crop potential of existing stand: ____ percent

Field replanted:

Estimated loss due to:

Late planting: ____ percent

Replanting: 13 percent

Sum of losses: ____ percent

Crop potential of a replanted crop: ____ percent

Alternate crops

If your herbicide program permits, you may decide to replant to soybean or another crop, especially after June 15.

Soybean is likely the best crop alternative, even though this probably means growing soybeans following a crop of soybeans. This is not usually recommended due to diseases, nematodes and yield considerations of continuous versus rotational cropping.

The only other crops that might give some production are forages, such as sudangrass. There are other factors to consider.

  • Do you need forage?

  • Do you have the equipment to harvest, handle and perhaps store forage?

  • What market opportunities are there?

If the crop is so badly damaged that it will not be economically feasible to pay the harvest costs, destroying the crop and planting a cover crop may be the best alternative.

It’s not good to leave the land fallow because of fallowing’s effect on the next crop. For instance, if the next crop is corn, phosphorus deficiency often occurs during the early vegetative growth periods for corn grown on fallow soil.

Salvaging a crop for silage

If hail strikes shortly before or after silking, it can drastically reduce grain yield. If live ear shoots remain on the plant, yield reduction will depend on the amount of leaf loss.

Kernel damage

Hail that occurs during grain filling can cause kernel damage. These kernels may just shrivel up or they may begin to rot, causing the entire ear to rot.

Molds often occur on kernels in damaged ears. Some molds can produce mycotoxins, which can affect animal performance, such as rate of gain, milk production and reproductive status.

Carefully inspect ears before feeding, and consult an animal nutritionist or veterinarian before feeding grain or silage with visible mold growth.

Smut

Smut galls may form on any corn plant tissue, but primarily occur on tassel or ear tissue. The organism may enter the plant when injured due to hailstones or insect feeding.

When smut develops, it significantly reduces grain development on individual plants and can cause breathing difficulty and discomfort to the combine operator during harvest.

Smut may also affect animal performance. If smut-infected corn is ensiled, consult an animal nutritionist or veterinarian before feeding. Even though ensiling reduces the smut’s toxic potential, infected plants may not be a good feed choice for some livestock.

Moisture

Silage made from corn plants with no ears or partially filled ears has 90 to 100 percent of the dry-matter value of silage made from well-eared plants.  

But because silage made from barren plants or stalks with little grain has higher moisture levels, it reduces total dry-matter intake and decreases animal performance.

Nitrates

Plants with little to no grain may have high levels of nitrates. The higher nitrate concentration occurs in the lower stalk, so cutting the plants at a higher level will decrease the nitrates of the harvested plantlage.

Fermentation after ensiling usually reduces the nitrate-nitrogen level to acceptable feeding levels.

Silo gases may be hazardous for the first 30 days following ensiling, so ensure the silo room is well-ventilated before entering it during this time period.

Stalk breakage

Stalk breakage may occur due to high-velocity winds.

Development stage

Stalk breakage can occur any time after corn plants have reached knee-high heights, but most frequently occur in the one-to-two-week window prior to tasseling. Plants at that stage are rapidly growing, so stalks are brittle and very vulnerable to breaking in high-velocity winds.

Breakage early in the growing season when plants are knee-high causes a reduction in stand, and the calendar date may make replanting not economically feasible.

Unbroken plants will compensate somewhat for reduced competition from adjacent plants, but a reduced plant population will lower the grain yield.

Breakage is also common just before tasseling. The effect of this injury on grain yield will depend on a number of factors:

  1. The percent of broken plants.

  2. The distribution of broken plants.

  3. The break’s location on the stalk.

  4. The crop’s growth stage.

Break location

Yields are least affected when the stalk breaks above the uppermost ear.

Plants adjacent to broken plants will partially compensate and produce more grain weight per plant due to less competition, especially for sunlight.

Even though the unbroken plants will produce more because of the lack of competition, it reduces yield for the entire field. The yield reduction averages 0.28 percent per percent of plants broken above the uppermost ear.

When the stalk breaks below the uppermost (top) ear, it reduces yields more than stalks broken above the top ear.

By the tassel stage, the second ear's potential size has already been determined, since the plant expected to fill kernels on the top ear. In most situations, the second ear produces little or no grain.

At this stage, the plant cannot adjust the number of kernels it can produce on the second ear. Research shows a yield reduction of 0.56 percent per percent of plants broken above the uppermost ear.

Dale R. Hicks, emeritus Extension agronomist and Seth L. Naeve, Extension agronomist

Reviewed in 2018

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